Liquid crystals have found use in a variety of electro-optical and display device applications, in particular those which require compact, energy-efficient, voltage-controlled light valves such as watch and calculator displays. Liquid crystal displays have a number of unique useful characteristics, including low voltage and low power of operation. In such displays, a thin layer of liquid crystal material is placed between glass plates and the optical properties of small domains in the layer is controlled by the application of electric fields with high spatial resolution. These devices are based upon the dielectric alignment effects in nematic, cholesteric and smectic phases of the liquid crystal compound in which, by virtue of dielectric anisotropy, the average molecular long axis of the compound takes up a preferred orientation in an applied electric field. However, since the coupling to an applied electric field by this mechanism is rather weak, the electro-optical response time of liquid crystal based displays may be too slow for many potential applications such as in flat-panel displays for use in video terminals, oscilloscopes, radar and television screens. Fast optical response times become increasingly important for applications to larger area display devices. Insufficient nonlinearity of liquid crystal based displays can also impose limitations for many potential applications.
Electro-optic effects with sub-microsecond switching speeds can be achieved using the technology of ferroelectric liquid crystals (FLCs) of N. A. Clark and S. T. Lagerwalll (1980) Appl. Phys. Lett. 36:899 and U.S. Pat. No. 4,367,924. These investigators have reported display structures prepared using FLC materials having not only high speed response (about 1,000 times faster than currently used twisted nematic devices), but which also exhibit bistable, threshold sensitive switching. Such properties make FLC based devices excellent candidates for light modulation devices including matrix addressed light valves containing a large number of elements for passive displays of graphic and pictorial information, optical processing applications, as well as for high information content dichroic displays.
Smectic C liquid crystal phases composed of chiral, nonracemic molecules possess a spontaneous ferroelectric polarization, or macroscopic dipole moment, deriving from a dissymmetry in the orientation of molecular dipoles in the liquid crystal phases (Meyer et al. (1975) J. Phys. (Les Ulis, Fr) 36:L-69). The ferroelectric polarization density is an intrinsic property of the material making up the phase and has a magnitude and sign for a given material under a given set of conditions. In ferroelectric liquid crystal display devices, like those of Clark and Lagerwall, appropriate application of an external electric field results in alignment of the chiral molecules in the ferroelectric liquid crystal phase with the applied field. When the sign of the applied field is reversed, realignment or switching of the FLC molecules occurs. This switching can be employed for light modulation. Within a large range of electric field strengths, the switching speed (optical rise time) is inversely proportional to applied field strength and polarization or dipole density (P), and directly proportional to orientational viscosity. Fast switching speeds are then associated with FLC phases which possess high polarization density and low orientational viscosity.
A basic requirement for application of ferroelectric liquid crystals in such devices is the availability of chemically stable liquid crystal materials which exhibit ferroelectric phases (chiral smectic C*) over a substantial temperature range about room temperature. In some cases, the ferroelectric liquid crystal compound itself will possess an enantiotropic or monotropic ferroelectric (chiral smectic C*) liquid crystal phase. Ferroelectric liquid crystal mixtures possessing chiral smectic C* phases with useful temperature ranges can also be obtained by admixture of chiral, nonracemic compounds, designated ferroelectric liquid crystal dopants, into a liquid crystal host material (which may or may not be composed of chiral molecules). Addition of the dopant can affect the ferroelectric polarization density and/or the viscosity of the C* phase and thereby affect the switching speed. Desirable FLC dopants are molecules which impart high ferroelectric polarization density to an FLC material without significantly increasing the orientational viscosity of the mixture.
While several useful ferroelectric liquid crystal materials (both pure compounds and mixtures) have thus been reported, optimum response times have not been achieved (theoretical limit estimated as 10-50 nsec. For this reason, new FLC materials particularly those having high polarization density and low viscosity are desirable, as are new FLC dopants which can impart desired properties to FLC materials. A useful property of FLC dopants is good miscibility in smectic C* matrix materials.
Ferroelectric liquid crystal (FLC) mixtures require that many criteria are met for use in commercial displays: e.g., wide temperature range of the SmC phase (−30° C.-80° C.), fast switching (<100 μs), optic axis rotation ˜45°, long C* and N* pitch, I-N-SmA-SmC phase sequence. Typically for FLC mixtures, ferroelectricity is achieved by doping an achiral, SmC host mixture with chiral dopants. Besides inducing spontaneous polarization to the mixture, the chiral dopants have the side effect of inducing pitch in both the nematic and SmC phases. The pitch must be balanced so that length of the helix is ˜4× greater than the thickness of the cell to ensure good alignment. Pitch compensators, i.e., additional chiral dopants with the opposite twisting power, are added to FLC mixtures for this purpose. Switching speed is a function of the spontaneous polarization and the viscosity for a given voltage. Since chirality is typically introduced to the mixture by branching the hydrocarbon tails of the constituent components, viscosity is also increased as the amount of chiral material is increased. Therefore, if it was possible to increase the spontaneous polarization without increasing the amount of chiral material, then one could avoid the shortcomings associated with increasing the viscosity and compensating for the pitch (and pitch compensators often act to increase viscosity themselves).